It is conventional in the commercial printings made by rotary printing apparatus of the blanket type to personalize the prints. This personalization is obtained by means of the tilting in operation from one printing group to another. The printing groups operate then in tandem, meaning that while one group is in the printing process, the other group is stopped in order to change the printing plate, and this without stopping the printing line which continues its operation. Frequent stops on such a printing line would actually be very damaging due to the loss of time and the waste of paper due to starting operations.
With such installations, when the printing group which is stopped has been equipped with a new printing plate and when the personalization printing is completed on the other printing group which is in use, a starting and setting in register under pressure of the merely equipped group is begun, while the pressure of the group which has completed a printing is released and unclutched in order to replace its printing plate. This type of printing machine has already been developed and generally involves typographic or flexographic printing techniques which are easy to put in practice. However, typographic and flexographic printing plates are costly, and require a photoengraving installation which the printers using offset apparatus usually do no possess. The present wish of printers is therefore that these so-called "tandem" machines are equipped with the offset printing process, the printing plates of which are cheap and easy to make. However, the carrying into effect of an offset printing is complex since it implies a delicate ink adjustment, a moistening setting (ink-water balance), an accurate setting of the touch, a preparation of the plates and blankets prior to the printing and an accurate setting of the stroke.